To explain it in a more detailed way, let us remember that the Volkov-Akulov model [1]}, as well as the de Sitter supergravity model [2]}, [3]}, involves a single chiral nilpotent superfield \(X\)  such that \(X^2=0\) , and supersymmetry is non-linearly realized. This theory, when formulated with a Lagrange multiplier superfield \(T\) , has a term in the superpotential of the form \(TX^2\)  as suggested in [4]}.
The equation of motion for this Lagrange multiplier is \(X^2=0\) .
Meanwhile in [5]} the authors made an unconventional step of promoting the Lagrange multiplier superfield \(T\)  to the status of a normal propagating superfield: such a theory has two superfields \(X\)  and \(T\)  with linearly realized supersymmetry.
As a result of this and several other unconventional modifications made in [5]}, the models they proposed suffer from a vacuum instability.
